U.S. patent application number 12/441128 was filed with the patent office on 2010-03-11 for control of power semiconductor devices.
This patent application is currently assigned to Cambridge Enterprise Limited. Invention is credited to Angus Toby Bryant, Patrick Reginald Palmer, Yalan Wang.
Application Number | 20100060326 12/441128 |
Document ID | / |
Family ID | 37232833 |
Filed Date | 2010-03-11 |
United States Patent
Application |
20100060326 |
Kind Code |
A1 |
Palmer; Patrick Reginald ;
et al. |
March 11, 2010 |
CONTROL OF POWER SEMICONDUCTOR DEVICES
Abstract
This invention relates to a control method and a circuit for
MOS-gated power semiconductor switching devices such as IGBTs or
MOSFETs, which allows control and optimisition of the current and
voltage commutation of a power semiconductor switching device and
freewheel diode pair in the basic half-bridge circuit found in a
wide range of equipment. The method comprises the stages of:
applying, upon receipt of a switch-on command signal, a voltage
function to the control terminal or the gate of the power
semiconductor switching device that allows a regulated current rise
in the device whilst maintaining the voltage across the device
falling at a predetermined rate; and at the instant when the
voltage across the diode begins to change from the on-state towards
the off-state level, applying a voltage function to the control
terminal or the gate of the power semiconductor switching device to
enable the voltage falling across the power semiconductor switching
device to track the voltage falling across the diode in order to
ensure a fast and controlled completion of the switching operation
without diode reverse voltage overshoot. The gate drive
automatically modifies the voltage function according to the
working condition thereby accounting for the actual operating
conditions.
Inventors: |
Palmer; Patrick Reginald;
(Cambridge, GB) ; Wang; Yalan; (Cambridge, GB)
; Bryant; Angus Toby; (Coventry, GB) |
Correspondence
Address: |
Clise, Billion & Cyr
605 US Hwy 169 North, Suite 300
Plymouth
MN
55441
US
|
Assignee: |
Cambridge Enterprise
Limited
Cambridge
GB
|
Family ID: |
37232833 |
Appl. No.: |
12/441128 |
Filed: |
September 10, 2007 |
PCT Filed: |
September 10, 2007 |
PCT NO: |
PCT/GB07/50531 |
371 Date: |
September 29, 2009 |
Current U.S.
Class: |
327/109 |
Current CPC
Class: |
H03K 17/166 20130101;
H03K 17/168 20130101; H03K 17/0828 20130101; H03K 17/08148
20130101 |
Class at
Publication: |
327/109 |
International
Class: |
H03K 17/082 20060101
H03K017/082 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2006 |
GB |
0617990.7 |
Claims
1. A method of controlling the commutation of a power semiconductor
switching device and freewheel diode pair, the method comprising:
applying, upon receipt of a switch-on command signal, a voltage
function to a control terminal or the gate of the power
semiconductor switching device, said voltage function allowing
regulation of a current rise in the device whilst changing the
voltage across the device; and substantially at the instant when
the voltage across the diode (V.sub.AK) begins to change from an
on-state towards an off-state voltage, applying a voltage function
to the control terminal or the gate of the power semiconductor
switching device to enable the voltage change across the power
semiconductor switching device to substantially track the voltage
change across the diode.
2. A method as claimed in claim 1 wherein said changing of said
voltage across said device comprises maintaining said voltage
falling according to a predetermined rate.
3. A method according to claim 1, further comprising synchronizing
a start of a reverse voltage appearing across the freewheel diode
with the start of the voltage fall of the power semiconductor
switching device when said diode begins switching and a current
peak in the power semiconductor switching device, such that current
commutation substantially without diode-reverse overshoot is
achieved.
4. A method according to claim 1, wherein said voltage function is
determined by the use of closed-loop feedback monitoring.
5. A method according to claim 1, further comprising altering said
voltage function to alter said commutation process.
6. A circuit for implementing a method according to claim 1, the
circuit comprising: a gate drive circuit for providing a gate drive
voltage for the power semiconductor switching device, and a control
signal generating circuit, connected to the gate drive circuit,
wherein upon receipt of a switch-on signal a first stage control
signal is produced to control the gate drive circuit to drive the
power semiconductor switching device to change said voltage across
said device and a second stage control signal is generated,
synchronized with the start of said voltage across said diode
falling, to control the gate drive circuit to drive the voltage
across the power semiconductor switching device to change
substantially in synchronism with the diode voltage.
7. A circuit according to claim 6, wherein said control signal
generating circuit comprises a closed-loop voltage feedback circuit
to feedback a sensed voltage from said power semiconductor
switching device and freewheel diode pair to said control signal
generating circuit, and a current monitoring circuit to feedback a
current sense signal from said power semiconductor switching device
and freewheel diode pair to said control signal generating circuit,
and wherein said first and second stage control signals are
responsive to said sensed voltage and said current sense
signal.
8. A circuit for controlling the commutation of a power
semiconductor switching device and freewheel diode pair, the
circuit comprising: means for applying, upon receipt of a switch-on
command signal a voltage function to a control terminal or the gate
of the power semiconductor switching device, a current rise in the
device whilst changing the voltage across the diode; and means for,
substantially at the instant when the voltage across the diode
begins to change from an on-state towards an off-state voltage,
applying a voltage function to the control terminal or the gate of
the power semiconductor switching device to enable the voltage
change across the power semiconductor switching device to
substantially track the voltage change across the diode.
9. A commutation control circuit for a chopper, said chopper
comprising an active switching device and a passive switching
device coupled in series, said commutation control circuit being
configured to control commutation of said switching devices such
that said switching devices switch substantially in synchronism,
said control circuit comprising: a first sense input to receive a
first sense signal from said chopper circuit; a second sense input
to receive a second sense signal from said chopper circuit; and a
control signal generating circuit coupled to said first and second
sense inputs and having an output to control switching of said
active switching device; and wherein said control signal generating
circuit is configured to use said first and second sense signals to
determine when said passive switch begins to switch and to control
said active switching device such that said active and passive
switching devices switch substantially in synchronism.
10. A commutation control circuit for a chopper, as claimed in
claim 9 wherein said first sense signal comprises a voltage sense
signal and said second sense signal comprises a current sense
signal.
11. A commutation control circuit for a chopper as claimed claim 9,
wherein one of said sense signals comprises a current sense signal
and wherein said control signal generating circuit is configured to
identify a feature on said current sense signal to determine when
said passive switch begins to switch.
12. A commutation control circuit for a chopper as claimed 11
wherein said feature comprises reversal of a current through said
passive switch.
13. A commutation control circuit for a chopper as claimed in claim
9 wherein said control signal generating circuit comprises at least
one digital control loop between one of said sense inputs and said
control output.
14. A commutation control circuit for a chopper as claimed in claim
9 wherein said active switching device comprises a MOS switching
device and wherein said passive switching device comprises a
rectifier.
15. The method according to claim 2, further comprising
synchronizing a start of a reverse voltage appearing across the
freewheel diode with the start of the voltage fall of the power
semiconductor switching device when said diode begins switching and
a current peak in the power semiconductor switching device, such
that current commutation substantially without diode-reverse
overshoot is achieved.
16. The method according to claim 15, wherein said voltage function
is determined by the use of closed-loop feedback monitoring.
17. The method according to claim 16, further comprising altering
said voltage function to alter said commutation process.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to the control of power
semiconductor devices, and in embodiments particularly relates to
the concurrent control of power semiconductor switching devices and
freewheel diodes.
BACKGROUND TO THE INVENTION
[0002] Power semiconductor devices are widely in use for a large
range of power applications, from low voltage chips, computers,
locomotives, to high voltage transmission lines. In most circuits,
freewheel diodes need to be used in relation with the power
semiconductor switching devices such as IGBTs (insulated gate
bipolar transistors) or MOSFETs (metal oxide semiconductor field
effect transistors) for a continuous load current conduction to
avoid large voltage damaging the semiconductor devices and the
circuit. High power IGBTs are often supplied as a module with
diodes that act as freewheel diodes within the module. IGBTs are
typically found in parallel with the diodes. Reliable switching
operation of both a power semiconductor switching device and a
freewheel diode is of primary importance. Optimisation of the
operation is also necessary to obtain high efficiency and low power
dissipation of the whole circuit. In order to achieve these aims,
one main concern is the current commutation from a freewheel diode
to a power semiconductor switching device during the switch-on
operation of the main switching device. Excessive diode voltage
overshoot and power dissipation associated with the diode reverse
recovery resulting from a poor commutation will impose high
stresses on both the devices, limiting their application range. In
particular, when there is a lack of good control, high power diodes
may suffer from punch-through and a resulting snappy recovery with
a sudden and large reverse voltage increase accompanied by
high-frequency oscillation.
[0003] Attempts have been made to control IGBTs and other power
semiconductor switching devices by use of feedback control
techniques including voltage, dv/dt, current and di/dt feedback
control. Open loop networks are also widely employed. However, the
freewheel diode cannot be directly controlled. Some lower power
circuits even dispense with the diodes and use controlled switching
devices in place of them. As a result, while those methods or gate
drives embody some of the techniques described above, they fail to
properly address the concurrent control of both the power
semiconductor switching device and the conventional freewheel
diode. They are frequently tailor-made to the application, where
the user has to ensure that the commutation results in transient
voltages and currents within bounds. Good control is difficult to
achieve in many applications as the conditions such as load
currents and temperatures are continuously changing. Even using
feedback control, the result is highly empirical, and the need to
adjust the gate drive for different operating conditions can make
conventional methods very undesirable and inefficient.
[0004] Background prior art can be found in WO 9743832 and US
2005253165.
SUMMARY OF THE INVENTION
[0005] According to a first aspect of the invention there is
provided a method of controlling the commutation of a power
semiconductor switching device and freewheel diode pair, the method
comprising the stages of: applying, upon receipt of a switch-on
command signal, a voltage function to a control terminal or the
gate of the power semiconductor switching device, said voltage
function allowing regulation of a current rise in the device whilst
changing the voltage across the device; and substantially at the
instant when the voltage across the diode (V.sub.AK) begins to
change from an on-state towards an off-state voltage, applying a
voltage function to the control terminal or the gate of the power
semiconductor switching device to enable the voltage change across
the power semiconductor switching device to substantially track the
voltage change across the diode.
[0006] The voltage function, that is the voltage shape or waveform,
is in embodiments chosen to control the rate of change of the
current and may comprise, a (slow) ramp or a (small) step, for
example of around 10 percent of the total change. By changing the
switching device voltage in this way the current through the diode
changes gradually (by contrast with a large step change in control
voltage). In embodiments parasitic or other inductance in the
circuit, more particularly in the series connected switching device
and diode, results in a gradual change of current. In embodiments
where voltage feedback is employed to control the switching an
approximately linear ramp of current may be achieved.
[0007] Broadly speaking in embodiments the concept is to change the
voltage on the control terminal or gate of the active switching
device using an initial ramp or step to determine when the passive
switching device (diode) begins to switch. At this point the
switching of the diode can be controlled by the active switch, thus
enabling the switching of the active (IGBT) and passive (diode)
switches to be synchronised. Thus embodiments of the method involve
watching the diode to determine when it begins to switch; this
point can be identified by identifying when the voltage across the
diode begins to fall towards or through zero. One can define a sign
convention such that when the diode is on the diode voltage is
called positive and when the voltage begins to fall below zero and
becomes negative the diode turns off--that is when the voltage
across the diode reverses. The skilled person will appreciate,
however, that the voltages across the active and passive switching
devices operate as a see-saw, so that as one rises the other falls,
and vice versa.
[0008] In embodiments changing the voltage across the power
semiconductor switching device comprises maintaining a falling
voltage, at a pre-determined rate, across the device. However
alternatively changing the voltage across this device may comprise
maintaining the voltage to follow a predetermined waveform which
may be stepped or linear (or piecewise linear) or both.
[0009] In embodiments the method comprises synchronising the start
of a voltage appearing across the freewheel diode (when the voltage
across the diode begins to fall towards or through zero) with the
main voltage fall of the power semiconductor switching device (that
is when the diode begins switching) and a current peak in the power
semiconductor switching device. In this way a safe and efficient
current commutation substantially without diode reverse overshoot
may be achieved. We illustrate later in reference to FIG. 4 when
the diode voltage (V.sub.AK) begins to switch (point T.sub.3 and
dashed line to corner 53) but it can be seen in this figure that
this point is not precisely coincident with the peak in diode
current I.sub.C. The peak in I.sub.C is where the rate of change of
current changes from positive to negative and thus is a
conveniently identifiable feature on the diode current waveform.
However it can be seen that this is not precisely coincident with
the switching point of the diode although it is substantially at
this instant (in embodiments we are slightly late).
[0010] In embodiments of the method the voltage function used to
control the rate of voltage decrease across the power semiconductor
switching device is determined by closed loop feedback monitoring.
In embodiments the voltage function for controlling the power
semiconductor switching device to track the diode voltage falling
can be altered to ensure an optimum commutation process. That is in
embodiments the voltage function (waveform) may be controlled to
control the commutation process.
[0011] In another aspect the invention provides a circuit for
implementing a method as described above, the circuit comprising: a
gate drive circuit for providing a gate drive voltage for the power
semiconductor switching device, and a control signal generating
circuit, connected to the gate drive circuit, wherein upon receipt
of a switch-on signal a first stage control signal is produced to
control the gate drive circuit to drive the power semiconductor
switching device to change said voltage across said device and a
second stage control signal is generated, synchronised with the
start of said voltage across said diode falling, to control the
gate drive circuit to drive the voltage across the power
semiconductor switching device to change substantially in
synchronism with the diode voltage.
[0012] As described above, in embodiments the voltages across the
diode and across the power semiconductor switching device operate
as a see-saw, that is the changes in voltage across these device
substantially track one another. (If absolute voltage polarities
are employed this may be expressed as both voltages falling at
substantially the same rate).
[0013] In embodiments the control signal generating circuit
comprises a closed loop voltage feedback circuit to feedback a
sensed voltage from the power semiconductor switching device and
freewheel diode pair to the control signal generating circuit, and
a current monitoring circuit to feedback a current sense signal
from the device and diode pair to the control signal generating
circuit. In embodiments both the first and second stage control
signals are responsive to the sensed voltage and sensed current
signal. In embodiments the sensed current signal senses a rate of
change of the current through the device and diode pair.
[0014] In a further related aspect the invention provides a circuit
for controlling the commutation of a power semiconductor switching
device and freewheel diode pair, the circuit comprising: means for
applying, upon receipt of a switch-on command signal a voltage
function to a control terminal or the gate of the power
semiconductor switching device, a current rise in the device whilst
changing the voltage across the diode; and means for, substantially
at the instant when the voltage across the diode begins to change
from an on-state towards an off-state voltage, applying a voltage
function to the control terminal or the gate of the power
semiconductor switching device to enable the voltage change across
the power semiconductor switching device to substantially track the
voltage change across the diode.
[0015] In a still further related aspect the invention provides a
commutation control circuit for a chopper, said chopper comprising
an active switching device and a passive switching device coupled
in series, said commutation control circuit being configured to
control commutation of said switching devices such that said
switching devices switch substantially in synchronism, said control
circuit comprising; a first sense input to receive a first sense
signal from said chopper circuit; a second sense input to receive a
second sense signal from said chopper circuit; and a control signal
generating circuit coupled to said first and second sense inputs
and having an output to control switching of said active switching
device; and wherein said control signal generating circuit is
configured to use said first and second sense signals to determine
when said passive switch begins to switch and to control said
active switching device such that said active and passive switching
devices switch substantially in synchronism.
[0016] In some preferred embodiments the commutation control
circuit identifies a feature in a current sense signal sensing a
current through the passive switching device, for example the
polarity change of the rate change of diode current (or, in more
accurately synchronised embodiments, the zero crossing of the diode
voltage), to determine when the passive switch begins to switch. In
embodiments the other sense signal comprises a voltage sense
signal. In principle two voltage sense signals could be employed
but this is less preferable because it is likely that for many
applications one of the sensed voltages would be a relatively high
voltage.
[0017] In embodiments the control signal generating circuit
comprises at least one digital control loop. For example a rate of
change of current may be sensed and used to generate a (1 bit)
timing control signal when the di/dt changes from positive to
negative (or vice versa). This may be used to identify,
approximately, a corner in the diode voltage, identifying a point
at which the diode commences to switch. The control signal
generating circuit may then generate a control signal to change the
voltage on the active switch, for example the collector or emitter
voltage of an IGBT, to bring this voltage down rapidly. In
embodiments this may be performed by using digital circuitry to
control a digital-to-analogue converter in response to the timing
control signal. This function may be performed, for example, by
high speed digital circuitry and implemented in an FPGA (field
programmable gate array). Often switching times are of order
nanoseconds and such an arrangement facilitates control on this
timescale. Such arrangements facilitate control of the switching
active and passive devices in synchronism.
[0018] We will therefore describe a control method and a circuit
for dynamically regulating switching operation of power
semiconductor switching devices such as IGBTs or power MOSFETs,
which addresses the aforementioned problems and allows control and
optimisation of the commutation of a power semiconductor switching
device and freewheel diode pair for a wide range of possible
operating conditions.
[0019] In embodiments the method comprises the stages of: applying,
upon receipt of a switch-on command signal, a voltage function to
the control terminal or the gate of the power semiconductor
switching device that allows a regulated current rise in the device
whilst maintaining the voltage across the device falling at a
predetermined rate; and at substantially the instant when the
voltage across the diode begins to change from the on-state towards
the off-state level, applying a voltage function to the control
terminal or the gate of the power semiconductor switching device to
enable the voltage falling across the power semiconductor switching
device to track the voltage falling across the diode in order to
ensure a fast and controlled completion of the switching operation
without diode reverse voltage overshoot.
[0020] Said instant is determined by specific real-time device
conditions instead of being preset. The gate drive automatically
modifies the voltage function according to the working condition
thereby accounting for the actual operating conditions. The
particular relationship between the power switching device voltage
and the diode voltage makes the method feature a so-called tracking
control.
[0021] By means of synchronising the start of the voltage appearing
across the freewheel diode with the main voltage fall of the power
semiconductor switching device during the switch-on process, the
method is able to avoid the reverse recovery voltage overshoot of
the freewheel diode and the high current spike of the power
semiconductor switching device. It can therefore achieve a safe and
efficient current and voltage commutation between the power
semiconductor switching device and the freewheel diode with
optimised switching losses and switching stresses below desired
limits
[0022] Said two-stage voltage function applied to the gate of the
power semiconductor switching device can be altered to ensure that
the optimum switch-on characteristics are achieved for the
particular switching device and diode being used and the
conditions. Multiple loop feedback and monitoring of several
variables such as current and voltage may be incorporated to
produce said two-stage voltage function. The various techniques
found in control theory may also be applied here with cascade
control of the gate voltage as well as the power semiconductor
switching device voltage, adaptive control of the features of said
two-stage voltage function and so forth.
[0023] In addition, the method is able to permit the regulation of
the current rise rate of the power semiconductor switching device
during its switch-on operation by circumscribing the voltage fall
across the switching device in most conventional circuits which
include stray inductance.
[0024] Consequently, the method achieves a reliable control of the
inherently non-linear transient behaviours of a power semiconductor
switching devices and a freewheel diode, and optimises the
performance of both devices in their SOAs (safe operating
areas).
[0025] Because of the controlled switching of both the power
semiconductor switching device and the freewheel diode, embodiments
of the present invention are also a good solution to serial device
operation for high power applications, wherein series connection of
power semiconductor switching devices such as IGBTs offers an
alternative to high voltage thyristors. While IGBTs connected in
series are each controlled in accordance with the present method,
the switch-on behaviour of individual switch device can be
synchronised and the interaction with diodes can be under control.
Hence, traditional bulky and expensive switch-on snubber circuit
for series connection can be avoided.
[0026] We also describe a circuit for implementing a method as
described above. In embodiments the circuit comprises: a gate drive
circuit for providing the gate drive voltage to the control
terminal or the gate of the power semiconductor switching device;
and a control signal generating circuit, connected to the gate
drive circuit, wherein, upon receipt of a switch-on signal, a first
stage of control signal is produced to control the gate drive
circuit to drive the power semiconductor switching device voltage
to fall at a predetermined rate, and at substantially the instant
when the diode voltage starts to change from the on-state towards
the off-state level a second stage of control signal is generated
to control the gate drive circuit to drive the power semiconductor
switching device voltage to fall at the same or other rate to the
diode voltage falling and to rapidly switch on the power
semiconductor switching device.
[0027] The control signal generating circuit may incorporate
closed-loop circuits and voltage or current monitoring
circuits.
[0028] Preferably said instant for the circuit is determined by the
change of the diode voltage, but it may instead be generated
according to other variables. A simple self-timing method to create
the two distinctive stages in said control signal is proposed in
the following embodiment. In this the switching device current is
monitored and the second stage of the switch-on is delayed until
the current in the switching device changes the polarity of its
rise rate. In embodiments this approach has the advantage of using
feedback loops, making control circuit self-contained and
compact.
[0029] The skilled person will appreciate that various circuit
approaches may be used for implementing the present method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Preferred embodiments of the invention will now be described
with reference to the accompanying drawings in which:
[0031] FIG. 1 is a diagram showing an embodiment of a circuit
according to the present invention employed in a half-bridge arm of
an inverter.
[0032] FIG. 2 shows typical switching waveforms during a hard
switch-on operation of an IGBT.
[0033] FIG. 3 is a diagram showing one specific circuit example of
the present invention and of the circuitry in FIG. 1 applied on an
IGBT and freewheel diode pair.
[0034] FIG. 4 shows the voltage function generated by the circuitry
of FIG. 1 or 3 along with resulting switching waveforms when the
method according to an embodiment of the present invention is
employed.
[0035] FIGS. 5 and 6 show the switching waveforms without the good
timing control which can be provided by embodiments of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0036] Practical details will now be described with reference to
the accompanying drawings.
[0037] FIG. 1 shows the circuitry of an embodiment of the present
invention applied in a half-bridge circuit, wherein two pairs of
power semiconductor switching devices (S.sub.1, S.sub.2) and
freewheel diodes (FWD.sub.1, FWD.sub.2) are connected as shown in
the figure. The half-bridge configuration is the basic topology
upon which most inverters are based. As shown in FIG. 1, a
freewheel diode is placed in anti-parallel with each power
semiconductor switching device. This arrangement normally
constitutes a power module. When the upper switching device S.sub.1
is on, the lower device S.sub.2 is off, and vice-versa. The diodes
FWD.sub.1 and FWD.sub.2 are to provide a path for the inductive
load current I.sub.L. The current interaction between S.sub.2 1 and
FWD.sub.1 2 is also shown. When the load current I.sub.L is flowing
in the direction as specified in FIG. 1, the current is commutated
from FWD.sub.1 2 to S.sub.2 1 during a switch-on operation of
S.sub.2 1. A gate drive circuit 3, connected to the gate terminal
G.sub.2 for controlling S.sub.2 1, is effectively a buffer circuit
for a control signal generating circuit 4. The DC loop stray
inductance L.sub.S, shown in FIG. 1, will also influence the
switching, as a current change in L.sub.S will induce a voltage
across it.
[0038] FIG. 2 shows a typical example of the current commutation
process between an IGBT in position S.sub.2 1 and its corresponding
freewheel diode FWD.sub.1 2 during a hard switching. Although the
following description is primarily with respect to IGBTs, it is
noteworthy that the circuit also suits other MOS-gated devices such
as power MOSFETs. As shown in FIG. 2, the voltage and current
waveforms associated with the IGBT 1 and the freewheel diode 2 are
usually depicted in five phases. During phase I, the IGBT
gate-emitter voltage V.sub.GE rises towards the gate threshold
voltage V.sub.TH, whilst the IGBT 1 is still off. As the IGBT
collector current I.sub.C begins to rise at the start of phase II,
the IGBT collector-emitter voltage V.sub.CE starts to fall at the
same time, because the rate of current change dI.sub.C/dt induces a
voltage across the inductance L.sub.S. Since the load current
I.sub.L is practically constant, when I.sub.C rises rapidly from
zero to the level I.sub.L, the diode current I.sub.A decreases from
its forward current, equal to I.sub.L, at an identical rate to
zero. The excess carrier stored in the diode drift region is
removed before the junction can become reverse biased. Thus I.sub.A
falls below zero to draw reverse recovery current and I.sub.C
increases above the level I.sub.L, reaching the peak
I.sub.L+I.sub.RR at the end of phase II, by which point the diode 2
has begun to regain reverse blocking capability. During phase III,
the growing reverse voltage across the diode 2 causes V.sub.CE to
decrease rapidly. With V.sub.CE approaching zero and I.sub.C
reducing, an overshoot V.sub.RR usually appears in V.sub.AK. The
relationship between V.sub.AK, V.sub.CE and I.sub.C is revealed in
the following equation:
V CE - V AK + L S .times. I C t = V DC ( 1 ) ##EQU00001##
[0039] The dI.sub.C/dt occurring during the reduction in I.sub.C to
the load current level I.sub.L in the absence of concurrent control
over V.sub.CE and V.sub.AK makes the diode voltage reverse
overshoot V.sub.RR very likely. V.sub.CE completes its drop to the
on-state voltage level in phase IV and phase V follows with the
growth of V.sub.GE to V.sub.GG(on).
[0040] Referring to FIG. 3, a circuit diagram shows an embodiment
of the present invention applied on an IGBT 1 and a freewheel diode
2. It comprises a gate drive circuit 3 and a control signal
generating circuit 4.
[0041] The gate drive circuit 3, comprising a high gain current
buffer stage 31, a MOSFET drive stage 32 and a gate resistance
R.sub.G 33, links the control signal generating circuit 4 and the
IGBT gate terminal G.
[0042] The control signal generating circuit 4 employs two feedback
loops from the device power side: the V.sub.CE voltage feedback
loop 41 and the di/dt feedback and conditioning loop 42. The
voltage feedback loop 41, which may be composed of an array of
resistors and capacitors, is attached to the collector terminal C
of the IGBT 1 and the non-inverting input of a very fast
high-bandwidth operational amplifier 43. This operational amplifier
43 compares a scaled V.sub.CE with a reference function V.sub.REF 5
connected to its inverting input. The reference function 5 is
generated by a high-speed FPGA (field programmable gate array)
digital controller 44 and a DAC (digital to analogue converter) 45
in a specific manner described below, although alternative
circuitry or microprocessor to perform the same function is
possible. In this embodiment a self-timing method is proposed and
realised via the di/dt feedback and conditioning circuit 42, which
comprises a measuring means to detect said instant that is used to
produce two distinctive stages according to the method and the
circuit. The emitter Kelvin inductance L.sub.E 6, between the IGBT
Kelvin terminal K and the emitter terminal E, may be used to fulfil
this purpose. A timing control signal 46 is generated in the di/dt
feedback and conditioning circuit 42 and fed back to the FPGA 44,
which accordingly produces changes on the reference profile
V.sub.REF 5. The FPGA 44 is connected through optical links or
alternative means to other controllers (not shown) that can provide
general on/off command signals.
[0043] According to an embodiment of the present invention, the
two-stage voltage function is provided by the control signal
generating circuit 4 to the IGBT gate terminal via the gate drive
circuit 3. Referring to FIG. 4, in this particular embodiment the
reference function V.sub.REF 5 is characterised by an initial
slowly-decreasing ramp 51 followed by a steeper ramp 52 and is in
essence the two-stage voltage function used according to the
method. The turning point 53 needs to coincide with said instant
when the diode voltage starts to change from the on-state towards
the off-state level. As observed in FIG. 2, when the IGBT current
reaches its peak, the diode voltage has just begun to go negative.
Furthermore, the current peak point actually happens when
dI.sub.C/dt changes from positive to negative, inducing a voltage
polarity change across the emitter Kelvin inductance 6.
Consequently, the self-timing method is proposed in this embodiment
to detect the current rate change instead of diode voltage change.
The timing requirement is fulfilled by the di/dt feedback and
conditioning circuit 42, wherein a fast comparator may be used to
monitor dI.sub.C/dt and detect the instant when the voltage across
the Kelvin inductance 6 changes the polarity. At this instant, the
di/dt feedback and conditioning circuit 42 generates a timing
control signal 46 to instruct the FPGA 44 to terminate the first
slow ramp 51 and to switch the reference function 5 to the second
steeper ramp 52. This process features a self-timing control and
therefore is the so-called self-timing method. Again alternative
methods or ways may be employed to perform embodiments of a method
according to the invention.
[0044] Under the regulation of the two-stage voltage function
V.sub.REF 5 according to an embodiment of the present invention,
the switching waveforms are optimised as shown in FIG. 4. At a time
zero, the IGBT 1 is off and blocks a voltage equal to V.sub.DC,
whilst the diode 2 is on with full current flow and a very low
on-state voltage across it. At time T.sub.1, the reference function
5 starts from a certain positive level V.sub.OFF 50 to a relatively
slow ramp 51. From time T.sub.2, the IGBT current I.sub.C rises
from zero and the collector-emitter voltage V.sub.CE falls to track
the reference ramp 51 according to a certain feedback ratio. At
time T.sub.3, when V.sub.AK starts to change towards the off-state
level, the second steeper ramp 52 is initiated and V.sub.CE is
directed to follow the diode voltage fall and to rapidly complete
switch-on operation. After V.sub.AK reaches -V.sub.DC at time
T.sub.4, the ramp 52 is terminated and the reference 5 is
maintained at a negative voltage level V.sub.ON 54 until a
switch-off command is given. From time T.sub.1 to T.sub.3, an
initial relatively slow ramp 51 circumscribes the decreasing rate
of V.sub.CE and slows down the current rise of the IGBT 1, which
results in lower current overshoots of both the IGBT 1 and the
diode 2. The main fall of the IGBT voltage V.sub.CE is delayed
until the diode 2 recovers and its voltage is ready to fall. Such
tracking reduces the voltage seen by the combination of the stray
inductance L.sub.S and the diode junction capacitance. As a good
synchronisation between the reference turning point 53 and the
instant of the diode voltage fall is maintained by the tracking
control, the decrease of V.sub.AK is a smooth reduction, which
completely avoids diode reverse voltage overshoot. A slight
inflexion in the middle of the diode voltage drop emerges, marking
a change of the voltage decreasing rate. This can be explained
according to Equation 1. While V.sub.CE, V.sub.AK and I.sub.C are
falling, V.sub.AK decrease at a faster rate than V.sub.CE because
of the changing voltage created across L.sub.S. When V.sub.CE has
dropped to a low value, I.sub.C enters a smooth decay period.
[0045] Preferably (but not essentially) good timing between the
instant when the diode voltage begins to change towards the
off-state level and the reference function turning point 53 is used
in the method to obtain an optimal performance. By comparison, two
scenarios without good timing control in this embodiment are shown
in FIG. 5 and FIG. 6. When the synchronisation between the
reference turning point 53 and the desired instant cannot be
realised, the optimal switching performance, as shown in FIG. 4,
will be lost. In FIG. 5, when the V.sub.REF turning point 53 is
lagging behind the start of the diode voltage change, there is a
high current overshoot and the diode reverse voltage overshoot
occurs too early, marking a kink in the middle of the diode
voltage. Although the diode voltage does not overshoot beyond
-V.sub.DC, both the current overshoot stress and switching power
losses are increased. When the turning point 53 is leading the
desired instant as shown in FIG. 6, the overshoot in the diode
voltage V.sub.AK reappears accompanied by a very high IGBT current
overshoot. The overall power losses may be slightly reduced
compared to that under the good timing control; however, the rating
margins in both current and voltage are largely reduced.
[0046] By use of embodiments of the present invention, controlled
switching of both the IGBT 1 and the freewheel diode 2
substantially removes commonly seen diode reverse voltage overshoot
and effectively reduces the current overshoots of both devices. The
power dissipation during the switching process is optimised. This
control process is a real-time self-optimisation independent of the
operating conditions. As the self-timing control stands, the
reference function 5 changes according to the real working
conditions. Furthermore, both dV.sub.CE/dt and dI.sub.C/dt of the
IGBT can be regulated through changing the ramp rate of the
reference function 5. Close tracking between the IGBT voltage and
the reference function 5 also allows devices in a series string to
commence switch-on simultaneously when connected in series.
[0047] The real-time optimisation concept and the circuit
configuration also hold for the switch-off operation of a power
semiconductor switching device. By monitoring the power states and
changing the reference function accordingly, a well-controlled
switch-off operation can also be realised.
[0048] The power semiconductor switching device may be an IGBT,
MOSFET, or a combination of such devices. The diode may be a
standard recovery diode, a fast recovery diode, and a wide range of
other types can also be used as freewheel diodes.
[0049] It will be understood that the invention is not limited to
the described embodiments and modifications can be made within the
spirit and scope of the claims appended hereto.
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